Modern broadband communications, such as hybrid fiber/coax (HFC), cable television (CATV), local multipoint distribution system (LMDS) and multiple multipoint distribution system (MMDS) requires high bandwidth not only in the downstream direction (from the headend to subscribers), but also in the upstream or return path direction (from each subscriber to the headend), as depicted in FIG. 1. The communication involved is digital in nature, though digital data involved may represent either analog or digital data, at its source.
Communication in the return path (upstream) for many of these systems is shared between many subscribers using time division multiple access (TDMA) communication, which involves transmission of data in discrete bursts as shown in FIG. 2. The headend receiver must therefore synchronize to short bursts (often as short as 8-64 bytes of eight bits each). In the sequence of bursts, consecutive bursts may be transmitted by different subscriber transmitters, each with a different, and time-varying, channel frequency response, as shown in FIG. 3. The high bandwidth requirements, together with channel artifacts including echoes, microreflections, multipath effects or other such impairments, lead to a need to adaptively equalize the received burst signals to mitigate intersymbol interference and/or fading.
Some modulation techniques, such as Quadrature Phase Shift Keying (QPSK), may require equalization only if symbol rates are high (above approximately 1,000,000 symbols/second), whereas higher order modulation, such as 16 QAM, may require equalization at even lower symbol rates to achieve near-optimal performance. In addition, in some systems narrow band interference is a major factor. Adaptive equalization has the added capability of removing, or at least attenuating, narrow band interferers.
Given these advantages of adaptive equalization, techniques are presented herein to implement the adaptive equalizer in a burst transmission environment. The following paragraphs describe specific techniques to process burst signals.
One of the challenges of burst transmission is how to quickly initialize the adaptive equalizer taps. In normal, continuous transmission systems, from one hundred to ten thousand symbols are used to train the equalizer. Other implementation of burst adaptive equalizers rely on long preambles (for example, 32 to 256 symbols) to adapt the equalizer. Still others require that equalizer coefficients be stored for each user and recalled before receiving a burst from a given user. The present invention accomplishes equalizer training using a short preamble, which may be as short as 16 symbols or less, and may be used with or without equalizer coefficient storage and recall.
According to the invention a digital burst mode receiver is provided for the reception of signals from multiple independent subscribers, as on an interactive cable television system adapted for subscriber inquiries and 2-way data transmission. Each burst comprises a preamble portion and a data portion. The received burst is sampled and fed to a sample storage buffer memory. The preamble portion's output is used to initialize an adaptive equalizer and to synchronize carrier and modulation clock. By repeating the processing of the preamble portion and optionally all or a portion of the data portion of each burst, accuracy of detection of the variable data is improved, so that it is feasible to receive individual bursts without burst-to-burst coordination and with greater efficiency resulting from shorter preambles.
One example of a communication system to which this invention is applicable is that described in U.S. Pat. No. 5,553,064, High Speed Bidirectional Digital Cable Transmission Systems, issued Sep. 3, 1996. This invention is an improvement upon and functionally extends the invention described in a pending application Burst Demodulator for use in High Speed Bidirectional Cable Transmission Systems (Krasner, Edwards, Xenakis and Currivan), of which the present applicant is a co-inventor.